Acid catalyzed polymerization

ABSTRACT

The present invention is an improvement in a free radical bulk polymerization process for preparing a high molecular weight polymer from a vinyl aromatic monomer characterized in that the polymerization is conducted in the presence of an acid catalyst having a pKa at 25° C. of less than 2, or salt thereof, wherein the improvement comprises dispersing the acid catalyst, or salt thereof, in a (meth)acrylic acid or ester thereof, prior to contact with the vinyl aromatic monomer. The (meth)acrylic acid or ester allows the acid catalyst to be dispersed within the vinyl aromatic monomer without causing cationic polymerization. The acid catalyst, or salt thereof, catalyzes the free radical polymerization reaction such that high molecular weight polymers are produced in reasonable reaction times.

CROSS REFERENCE STATEMENT

This application claims the benefit of U.S. Provisional Application No.60/050,829, filed Jun. 26, 1997.

The present invention relates to an improved process for producing highmolecular weight polymers from vinyl aromatic monomers.

BACKGROUND OF THE INVENTION

High molecular weight vinyl aromatic polymers, particularly polymershaving weight average molecular weights (Mw) of greater than 300,000,have been typically produced by anionic polymerization rather than byfree radical polymerization due to the slow polymerization rates used infree radical techniques to achieve high molecular weight polymers.However, anionic polymerization processes require expensive anionicinitiators, such as organolithium compounds, and tend to producediscolored products due to the presence of residual lithium-containingsalts.

High molecular weight vinyl aromatic polymers have also been produced byfree radical polymerization in the presence of a soluble organic acidhaving pKa of 0.5 to 2.5, as in U.S. Pat. Nos. 5,145,924 and 5,115,055.However, in these processes the acid does not bind to the polymer andcan migrate from the polymer during use, which can cause corrosion ofmold surfaces. Additionally, such strong acids are difficult to dispersein vinyl aromatic monomers without initiating cationic polymerizationand producing low molecular weight fractions.

Copending application 08/606,182 by Priddy, et al., discloses a processfor preparing a high molecular weight polymer from a vinyl aromaticmonomer using free radical polymerization, characterized in that thepolymerization is conducted in the presence of a vinyl acid having a pKaat 25° C. from about 0.1 to about 2.0. The vinyl acid catalyzes thepolymerization reaction such that high molecular weight polymers areproduced in reasonable reaction times and the C--C double bonds of thevinyl acid react with the vinyl aromatic monomer such that the vinylacid is not free to migrate from the polymer. However, such vinyl acidsare also difficult to disperse in vinyl aromatic monomers.

Therefore, there remains a need for an acid catalyzed free radicalpolymerization process for producing high molecular weight polymers inreasonable reaction times from vinyl aromatic monomers using freeradical polymerization without the disadvantages of the prior art.

SUMMARY OF THE INVENTION

The present invention is an improvement in a free radical bulkpolymerization process for preparing a high molecular weight polymerfrom a vinyl aromatic monomer characterized in that the polymerizationis conducted in the presence of an acid catalyst having a pKa at 25° C.of less than 2, or salt thereof, wherein the improvement comprisesdispersing the acid catalyst or salt thereof, in a (meth)acrylic acid orester thereof, prior to contact with the vinyl aromatic monomer. The(meth)acrylic acid or ester thereof allows the acid catalyst to bedispersed within the vinyl aromatic monomer without causing cationicpolymerization. The acid catalyst or salt thereof catalyzes the freeradical polymerization reaction such that high molecular weight polymersare produced in reasonable reaction times.

The high Mw polymer produced by the process of the present invention canbe employed in applications where high molecular weight vinyl aromaticpolymers are suitably used, such as foam sheet, films and injectionmolding processes. They can also be combined with polymers of differingMw to make polymer compositions having a multimodal molecular weightdistribution, preferably a bimodal molecular weight distribution,hereinafter referred to as bimodal compositions.

In another aspect of the present invention, a bimodal composition isproduced containing a high molecular weight polymer from the firstaspect of the present invention and a lower molecular weight polymer ofa vinyl aromatic monomer.

DETAILED DESCRIPTION OF THE INVENTION

Vinyl aromatic monomers for use according to the present inventioninclude, but are not limited to, those vinyl aromatic monomerspreviously known for use in polymerization processes, such as thoseexemplified in U.S. Pat. Nos. 4,666,987, 4,572,819 and 4,585,825.Preferably, the monomer is of the formula: ##STR1## wherein R ishydrogen or methyl, Ar is phenyl, halophenyl, alkylphenyl oralkylhalophenyl, wherein any alkyl group contains 1 to 6 carbon atoms.The term halophenyl refers to a phenyl substituted with one or twohalogen atoms, the term alkylphenyl refers to a phenyl substituted withone or two alkyl groups, and the term alkylhalophenyl refers to phenylsubstituted with one or two alkyl groups which contain a halogensubstituent or to a phenyl substituted with a halogen and an alkylsubstituent. More preferably Ar is phenyl or alkylphenyl with phenylbeing most preferred. In addition, the polymerization may be conductedin the presence of predissolved elastomer to prepare impact modified, orgrafted rubber containing products, examples of which are described inU.S. Pat. Nos. 3,123,655, 3,346,520, 3,639,522, and 4,409,369.

The acid catalyst used in the process of the present invention may beany acid having a pKa at 25° C. of less than 2, or salt thereof.Accordingly, throughout the application, any teachings referring to theacid catalyst can also be applied to a salt thereof. The pKa is used toexpress the extent of dissociation of acids in water, and is thenegative logarithm (to the base 10) of the equilibrium constant, Ka.Such acid catalysts include but are not limited to2-sulfoethyl-methacrylate (SEM), acryloamidopropanesulfonic acid (AMPS),2-sulfopropylmethacrylate, methane sulfonic acid, camphorsulfonic acid,p-toluenesulfonic acid, phosphoric acid, sulfuric acid, or mixturesthereof. Representative salts include 2-fluoro-1-methylpyridiniumtosylate, SEM lithium salt, and 2 sulfoethyl-tetrabutyl ammonium salt.Preferably the acid catalyst is a vinyl functional sulfonic or vinylfunctional phosphonic acid such as 2-sulfoethylmethacrylate (SEM),vinylphosphonic acid (VPA), 2-sulfopropylmethacrylate (SPM),styrenesulfonic acid (SSA), styrene-phosphonic acid (SPA),4-vinylbenzylphoshonic acid (VBPA), 2-sulfoethylacrylate (SEA),α-phenylvinylphosphonic acid (PVPA), or mixtures thereof, with the mostpreferred vinyl acid being SEM. These acids are known and arecommercially available or can be made by processes as described in U.S.Pat. No. 4,529,559 which is incorporated herein by reference.

The acid catalyst is dispersed in a (meth)acrylic acid or ester thereof,before combining with the vinyl aromatic monomer. The term (meth)acrylicacid refers to either a methacrylic acid or an acrylic acid. A(meth)acrylic acid ester would be any C₁ -C₈ ester of methacrylic acidor acrylic acid. Accordingly, throughout the application, any teachingsreferring to (meth)acrylic acid can also be applied to an ester thereof.The (meth)acrylic acid acts as a reactive dispersant, copolymerizinginto the vinyl aromatic polymer chain during polymerization such that itdoes not contaminate the polymer or the volatile recycle stream. The(meth)acrylic acid also serves as a buffer for very strong acidcatalysts such that they can be added to styrene without initiatingcationic polymerization. The acid catalyst generally comprises from 0.1to 75 weight percent of the acid catalyst/(meth)acrylic acid mixture,typically from 0.5, preferably from 1, more preferably from 5, and mostpreferably from 10 to 75, typically to 70, preferably to 65, morepreferably to 60, and most preferably to 50 weight percent of the acidcatalyst/(meth)-acrylic acid mixture.

The amount of acid catalyst/(meth)acrylic acid mixture present in thepolymerization is dependent upon the concentration of the acid catalystin the mixture. Typically, the acid catalyst is present in thepolymerization in amounts such that a high molecular weight polymer isproduced without appreciably adversely affecting the properties of thepolymer. The amount of acid catalyst needed will depend upon theparticular acid catalyst used. It has been found that good results areobtained when the ratio (pKa X acid catalyst molecularweight)/(concentration of the acid catalyst in ppm based on vinylaromatic monomer) is from 0.01, more preferably from 0.05, mostpreferably from 0.08, to 1, more preferably to 0.5, and most preferablyto 0.3. In the case of acid salts, this would be based on the pKa of theacid component of the salt. In general, acid catalysts of higher pKavalues will be present in greater amounts than acid catalysts of lowerpKa values. Generally, the acid catalyst will be present in an amount offrom 10, typically from 25, preferably from 30, more preferably from 40and most preferably from 50 ppm to 1000, typically to 950, preferably to900, more preferably to 850 and most preferably to 800 ppm, based on theamount of vinyl aromatic monomer. Sulfur containing vinyl acids can beused in amounts which will produce a high molecular weight polymerwithout initiating cationic polymerization. If the amount of sulfurcontaining vinyl acid is too great, the acid will initiate cationicpolymerization which will produce low molecular weight polymers, e.g.less than 20,000 Mw. Cationic polymerization can therefore be detectedby the formation of low molecular weight fractions within the highmolecular weight polymer produced. Typically, amounts of from about 10to about 500 ppm of a sulfur containing vinyl acid is present,preferably from about 15 to about 400, more preferably from about 20 toabout 300, and most preferably from about 25 to about 200 ppm based onthe amount of vinyl aromatic monomer. Phosphorus containing vinyl acidsmay be present in larger amounts and are not known to initiate cationicpolymerization. Typically, amounts of from about 500 to about 20,000 ppmof phosphorus containing vinyl acid is present, preferably from about600 to about 15,000, more preferably from about 800 to about 10,000 andmost preferably from about 1000 to about 5000 ppm based on the amount ofvinyl aromatic monomer.

The polymerization can optionally be conducted in the presence of anitroxyl stable free radical compound as described in "NarrowPolydispersity Polystyrene by a Free-Radical Polymerization Process-RateEnhancement", Macromolecules 1994, 27, pg. 7228-7229. Typical nitroxylradical compounds include 2,2,6,6-tetramethyl-1-piperidinyloxy and3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy. Typical amounts ofnitroxyl stable free radical are from 10 ppm to 2000 ppm based on theamount of vinyl aromatic monomer.

An initiator may optionally be present in the free radicalpolymerization. Typical initiators include azo compounds and peroxides.Exemplary peroxides include tert-butylperoxybenzoate,tert-butylperoxyoctoate, di-tert-butylperoxide, dibenzoylperoxide,dilauroylperoxide, 1,1-bis-tert-butylperoxycyclohexane,1,1,-bis-tert-butylperoxy-3,3,5-trimethylcyclohexane anddicumylperoxide.

The polymerization can be conducted at any temperature at which a highmolecular weight polymer will be produced. Suitable polymerizationtemperatures are from about 80° C. to about 170° C., preferably fromabout 110° C. to about 160° C., with about 115° C. to about 150° C.being the most preferred.

The amount of time needed for the polymerization is dependent upon anumber of factors including the acid concentration, the (optional)initiator concentration, the (optional) nitroxyl radical concentration,the percent conversion desired and the reaction temperature. Typically,the polymerization is conducted from 0.5 to 8 hours, preferably from 1to 6 hours and most preferably from 1 to 5 hours.

The molecular weight of the resulting polymer is dependent upon a numberof factors including the temperature, the (optional) initiatorconcentration, the (optional) nitroxyl radical concentration, the acidconcentration and the time of reaction. The term molecular weight (Mw)refers to the weight average molecular weight as determined by gelpermeation chromatography. The molecular weight of the high molecularweight polymer formed according to the process of the present inventionis from 100,000 to 450,000, most preferably from about 350,000 to about450,000. Polymers having molecular weights greater than 450,000 areundesirable because they are difficult to process.

The polymers produced by the first aspect of the present invention canalso have the added advantage of a narrow polydispersity. Polydispersityrefers to the ratio of the weight average molecular weight to the numberaverage molecular weight. The high molecular weight polymers produced bythe process of the present invention typically have polydispersityvalues of less than 2.5.

The high molecular weight polymers produced according to the process ofthe present invention may be blended with other ingredients, such asmold release additives, lubricants, colorants, ignition resistantadditives, impact modifiers, glass fibers, and other polymers such aspolyphenylene oxides, polycarbonates, elastomeric copolymers such asstyrene-butadiene block copolymers, polybutadiene, etc., as well asother polyvinylaromatic resins.

The high molecular weight vinyl aromatic polymer produced according tothe process of the present invention may be recovered by removingdiluent and/or unreacted monomer from the reaction mixture resultingfrom the polymerization. Alternatively, the reaction mixture can befurther processed to produce polymodal compositions containing polymersof differing molecular weight.

In one embodiment, bimodal compositions can be produced as described inU.S. Pat. No. 4,585,825 by Wesselmann, which is incorporated herein byreference. The bimodal composition is made by adding initiator to thehigh molecular weight polymer/unreacted monomer mixture, andpolymerizing the unreacted monomer to produce a relatively low molecularweight vinyl aromatic polymer in the presence of the high molecularweight polymer.

The initiator may be any initiator or mixture of initiators which willpolymerize the unreacted monomer in the mixture containing the highmolecular weight polymer. The initiator can be any free radicalinitiator as discussed previously, and is preferably a peroxideinitiator such as tert-butylperoxybenzoate, tert-butylperoxyacetate,di-tert-butylperoxide, dibenzoylperoxide, dilauroylperoxide,1,1-bis-tert-butyl peroxycyclohexane,1,1,-bis-tert-butylperoxy-3,3,5-trimethylcyclohexane anddicumylperoxide. Typical amounts of initiator are from 10 ppm to 2000ppm based on the amount of vinyl aromatic monomer.

In another embodiment of preparing bimodal compositions, the initiatorand acid catalyst/(meth)acrylic acid mixture can be added to the initialmonomer feed prior to the production of high molecular weight polymer.In this process, the initiator is chosen such that lower molecularweight polymer is produced first. Upon depletion of the initiator,further polymerization yields a high molecular weight polymer. Theinitiator used in this embodiment preferably has a one hour half lifetemperature of less than 120° C. The one hour half life temperature isthe temperature at which one half of the initiator is consumed after onehour of reaction time. Suitable initiators include benzoylperoxide,1,1-bis(t-butylperoxy)cyclohexane, 2,2-bis-(4,4-di- t-butylperoxy!cyclohexyl)propane, and t-butylperoxypivalate. Typical amounts of theseinitiators are from 10 ppm to 2000 ppm based on the amount of vinylaromatic monomer.

Optionally, chain transfer agents may be utilized in the preparation ofthe lower molecular weight polymer. Suitable chain transfer agentsinclude common chain transfer agents known in the art such asmercaptans. Preferably, the chain transfer agent is n-dodecylmercaptanor terpinoline. Typical amounts of chain transfer agents are from 10 ppmto 4000 ppm based on the amount of vinyl aromatic monomer.

The desired Mw of the high molecular weight polymer in polymodalcompositions will be significantly higher than the desired Mw describedin the first aspect of the present invention, since the high Mw polymerwill be blended with a lower molecular weight polymer. In this aspect ofthe present invention the Mw can be selected according to the desired Mwof the polymodal composition and is preferably from about 300,000 toabout 2,000,000 more preferably from about 350,000 to about 1,500,000,and most preferably from about 400,000 to about 800,000.

The desired Mw of the lower molecular weight polymer is also a matter ofchoice and is dependent upon the desired Mw of the bimodal compositionand the desired properties. Preferably the Mw is from about 50,000 toabout 200,000.

The amount of high molecular weight polymer present in the polymodalcomposition can be selected according to the desired properties of thepolymodal composition. Typically, from 1 to 40 percent, preferably from5 to 35 percent, and most preferably from 10 to 20 percent of the highmolecular weight polymer is present.

The average Mw of polymodal compositions is dependent upon the Mw of thepolymers contained within the composition. The average Mw of the bimodalcomposition of this embodiment of the present invention is preferablyfrom about 120,000 to about 600,000, more preferably from about 130,000to 500,000, and most preferably from about 140,000 to 400,000.

The composition containing both high and lower molecular weight vinylaromatic polymers may be any combination of two vinyl aromatic polymersbut is preferably a blend of polymers having the same composition (i.e.,homopolymers of the same monomeric units or copolymers having the samecomonomeric units in the similar ratios). More preferably, both polymersare polystyrene.

A preferred process for the production of the bimodal composition is acontinuous polymerization process wherein a group of several distinctreaction zones within one or more reactors are used in series to createthe different molecular weight polymers. The different zones aremaintained at the desired temperatures and supplied with the appropriatereactants necessary to produce the desired amounts of polymer having thespecified molecular weights, such that polymodal compositions areproduced.

In a preferred process, wherein a high molecular weight polymer isproduced first, an earlier polymerization zone is maintained such that ahigh molecular weight polymer is produced, while a later zone receivesthe reaction mixture from the previous zone, including the highmolecular weight polymer. The later zone is optionally supplied withadditional reactants, including an initiator and/or a chain transferagent, and is otherwise maintained such that it produces a lowermolecular weight polymer in the presence of the previously produced highmolecular weight polymer, a blend of the two components being achievedthereby. Usually, the earlier high molecular weight polymer-producingreactor or zone is at a lower temperature than the subsequent lowermolecular weight polymer producing reactor or zone.

In another preferred process, wherein the lower molecular weight polymeris produced first, an earlier zone is maintained such that lowermolecular weight polymer is produced, while a later zone receives thereaction mixture from the previous zone including the lower molecularweight polymer. The later zone is maintained such that it produces highmolecular weight polymer in the presence of the previously producedlower molecular weight polymer, a blend of the two components beingachieved thereby. Both zones may be maintained at the same temperatureor the later zone may be at a higher temperature than the previous zone.

Bimodal compositions containing high molecular weight and lowermolecular weight polymers are useful for a variety of applicationsincluding foam board, foam sheet, injection molding, and extrusion. Thefollowing examples are provided to illustrate the present invention. Theexamples are not intended to limit the scope of the present inventionand they should not be so interpreted. Amounts are in weight parts orweight percentages unless otherwise indicated.

EXAMPLES

The following mixtures are prepared:

Solution A

2.5% by weight concentrated sulfuric acid (H₂ SO₄) dissolved inmethacrylic acid.

Solution B

10% w/w ethylbenzene in styrene and 300 ppm of 1,1-bis(t-butylperoxy)cyclohexane.

Solution C

Ten grams of Solution B and 0.01 grams of pure methacrylic acid (MAA).

Solution D

Ten grams of Solution B and 0.01 grams of Solution A.

No color change or other evidence of polymerization are observed insolutions B, C or D. Two grams of Solutions B, C, and D are each loadedinto glass ampoules. The ampoules are sealed under vacuum and placed inan oil bath heated at 110° C. The bath is heated to 175° C. over a fourhour period and the ampoules are removed from the oil bath. The percentpolystyrene in the solutions is determined by placing a portion of thepolymer syrup in a vacuum oven for 10 minutes at 225° C. The molecularweight of the polystyrene produced is determined by analyzing a portionof the syrup from each ampoule using gel permeation chromatography.Solutions B and C are comparative and Solution D is an example of theinvention.

    ______________________________________                                        Solution           % Solids Mw                                                ______________________________________                                        B (No Catalyst)    80       218                                               C (MAA)            81       217                                               D (H.sub.2 SO.sub.4 and MAA)                                                                     74       290                                               ______________________________________                                    

The results show that MAA has no effect on styrene polymerizationwhereas sulfuric acid dispersed in MAA results in the formation of highmolecular weight polystyrene.

What is claimed is:
 1. In a free radical bulk polymerization process forproducing high molecular weight polymers from a vinyl aromatic monomercharacterized in that the polymerization is conducted in the presence ofa acid catalyst having a pKa of less than 2 at 25° C., or a saltthereof, in a sufficient amount such that a high Mw polymer is producedwithout initiating cationic polymerization, an improvement wherein theacid catalyst is first dispersed in a (meth)acrylic acid or esterthereof, prior to contact with the vinyl aromatic monomer.
 2. Theprocess of claim 1 wherein the amount of acid catalyst or salt thereofis from 10 to 1000 ppm based on the amount of vinyl aromatic monomer. 3.The process of claim 2 wherein the acid catalyst or salt thereof is,2-sulfopropylmethacrylate, methane sulfonic acid, camphorsulfonic acid,p-toluenesulfonic acid, 2-fluoro-1-methylpyridinium tosylate,vinylphosphonic acid (VPA), acrylamidopropanesulfonic acid (AMPS),styrenesulfonic acid (SSA), styrenephosphonic acid (SPA),4-vinylbenzylphoshonic acid (VBPA), 2-sulfoethylacrylate (SEA), orα-phenylvinylphosphonic acid (PVPA).
 4. The process of claim 3 whereinthe acid catalyst is 2-sulfoethylmethacrylate.
 5. The process of claim 4wherein the 2-sulfoethyl-methacrylate is first dispersed in methacrylicacid or methylmethacrylate.
 6. The process of claim 4 wherein the2-sulfoethyl-methacrylate is first dispersed in acrylic acid orbutylacrylate.
 7. The process of claim 1 wherein the vinyl aromaticmonomer is styrene.
 8. The process of claim 1 wherein the acid catalystis a vinyl acid.
 9. The process of claim 1 wherein the polymerization isconducted in the presence of an initiator.
 10. The process of claim 9wherein the initiator is tert-butylperoxybenzoate,tert-butylperoxyoctoate, di-tert-butylperoxide, dibenzoylperoxide,dilauroylperoxide, 1,1-bis-tert-butylperoxycyclohexane,1,1,-bis-tert-butylperoxy-3,3,5-trimethylcyclohexane or dicumylperoxide.11. The process of claim 1 wherein the polymerization is conducted inthe presence of a nitroxyl stable free radical.
 12. The process of claim11 wherein the stable free radical is2,2,6,6-tetramethyl-1-piperidinyloxy or3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy.
 13. The process ofclaim 1 wherein a portion of the vinyl aromatic monomer has beenpolymerized in the presence of a free radical initiator such that alower molecular weight polymer is produced prior to the production ofhigh Mw polymer.
 14. The process of claim 13 wherein the high molecularweight vinyl aromatic polymer and the lower molecular weight vinylaromatic polymer are both polystyrene.
 15. The process of claim 14wherein the high molecular weight polystyrene has a Mw of about 350,000to about 2,000,000 and the lower molecular weight polystyrene has a Mwof about 50,000 to about 200,000.
 16. The process of claim 13 whereinthe initiator is tert-butylperoxybenzoate, tert-butylperoxyoctoate,di-tert-butylperoxide, dibenzoylperoxide, dilauroylperoxide,1,1-bis-tert-butylperoxycyclohexane, 1,1,-bis-tert-butylperoxy-3,3,5-trimethylcyclohexane or dicumylperoxide.
 17. The process of claim 13wherein the lower Mw polymer is produced in the presence of a chaintransfer agent.
 18. The process of claim 17 wherein the chain transferagent in n-dodecylmercaptan.
 19. The process of claim 13 wherein thecomposition of high molecular weight polymer and lower molecular weightpolymer has combined average Mw of about 100,000 to about 600,000. 20.The process of claim 1 wherein the vinyl aromatic monomer is partiallypolymerized to produce a mixture of the high molecular weight polymerand unreacted monomer, a free radical initiator is added to said mixtureand the unreacted vinyl aromatic monomer is polymerized under conditionssuch that a lower molecular weight polymer is produced.
 21. The processof claim 20 wherein the high molecular weight vinyl aromatic polymer andthe lower molecular weight vinyl aromatic polymer are both polystyrene.22. The process of claim 21 wherein the high molecular weightpolystyrene has a Mw of about 350,000 to about 2,000,000 and the lowermolecular weight polystyrene has a Mw of about 50,000 to about 200,000.23. The process of claim 20 wherein the initiator istert-butylperoxybenzoate, tert-butylperoxyacetate,di-tert-butylperoxide, dibenzoylperoxide, dilauroylperoxide,1,1-bis-tert-butylperoxycyclohexane,1,1-bis-tert-butylperoxy-3,3,5-trimethylcyclohexane or dicumylperoxide.24. The process of claim 20 wherein the lower Mw polymer is produced inthe presence of a chain transfer agent.
 25. The process of claim 24wherein the chain transfer agent in n-dodecylmercaptan.
 26. The processof claim 20 wherein the composition of high molecular weight vinylaromatic polymer and lower molecular weight vinyl aromatic polymer has aMw of about 100,000 to about 600,000.